The ins and outs of standard fluoroplastic convoluted tubing

REPRINTED FROM THE IAPD MAGAZINE

Convolutions are molded into PTFE Convo-Tex™ whereas Convo-Flex™ is constructed by creating annular rings with a unique angle.

When it comes to fluoroplastic convoluted tubing it can be very confusing just what you are getting. Do you want standard and if so, what is standard? Is it PTFE, FEP or PFA? Is it annular or helical? For each user, “standard” depends on the user’s application and preferences.

In the early 1960s, fluoroplastic convoluted tubing was originally developed to provide a light-weight, non-flammable, corrosion resistant alternative to the heavier metal interconnects in aircraft. Later on, when the fluid handling industry became interested in using fluoroplastic convoluted tubing, the application re­quire­ments began to change in regard to flexibility, color, wall thickness, and how to attach the tubing to a compression fitting. As these issues were addressed, many new variations of fluoroplastic convoluted tub­ing were developed.

Around 1980, one of the new variations was fluoroplastic corrugated tubing. Unlike convoluted tubing, where the convolutions are molded into the tube, corrugations were actually made by creating an annular ring with a unique angle that allows for extension and compression and the tightest possible bend radius. The result was a flexible tube that could handle an extremely tight bend radius and goose neck around obstacles without collapsing the tube. This was not possible with convoluted constructions.

In those early years, when fluoroplastic convoluted tubing and corrugated were both supplied in annular form, corrugated tubing was sometimes incorrectly referred to as “convoluted tubing.” Annular tubing or corrugated tubing is referred to as “rings” of tubing formed into a continuous tube. If you cut this tube apart you could cut it on the valley or peak and you would have a straight perpendicular cut. If you cut through a valley on one side you will be in a valley on the other side. However, over the years, most of the fluoroplastic convoluted tubing evolved into a helical convolution. Helical convoluted tubing is a spiraled convoluted tubing. If you cut the tubing perpendicular you will probably cut a peak on one side and a valley on the other.

Convoluted tubing uses twice the material of a standard smooth bore tube, creating a tube reinforced within itself to handle higher pressures and offering increased flexibility. Convoluting the tube increases the flexibility of a tube by adding hoop strength to the circumferential strength to resist collapse, which results in a tube capable of turning corners and winding between machinery. It can be supplied in coils (sometimes as long as 1,000 feet continuous) or cut to length with formed cuffs on each end. These cuffs al­low the convoluted tubing to be attached to machinery or fittings without comprising the integrity of the tube and ensuring a sure fit.

Today, almost all convoluted tubing is created by machines. In PTFE, the smooth bore tubing is run through an automatic convoluter that heats the tubing back up into the gel state and then pushes the soft material through a die to create the convolutions. It takes approximately 100 feet of smooth bore tubing to create 50 feet of convoluted tubing.

View the complete article online.

This article was written by Janine Kruit, Texloc/Atlantic Tubing, a Division of Parker Hannifin Corporation.

PTFE — fluoropolymer
workhorse
Characteristics and applications for PTFE.

Diversity in PTFE
Fillers provide different application advantages.

About PTFE
Properties of this versatile material.

Test your knowledge
What do you know about PTFE?

Online plastic resources
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PTFE — fluoropolymer workhorse

REPRINTED FROM THE IAPD MAGAZINE

The fluorocarbon resins are an outgrowth of the fluorine, carbon and chlorine compounds developed during the 1930s, as part of the same research which developed refrigerants. Dr. Roy Plunkett’s research led to one of today’s most widely used fluoropolymers — polytetrafluoro­ethylene, better known as PTFE.

Properties and characteristics
While classified as a thermoplastic resin, PTFE does not melt and flow. It has a crystalline structure at normal temperatures but when heated above 620°F (326.6°C), it becomes an amorphous transparent gel. (“Fluorocarbons” by Merritt A. Rudner, published 1958.)

PTFE has an impressive array of physical properties which make it the optimum material of choice for applications ranging from wire and cable insulation to medical catheter linings to active wear fabric. Those properties include:

• Lowest coefficient of friction
• Excellent low loss for electrical applications
• Superior heat resistance
• Inherently UV resistant
• Excellent chemical resistance
• FDA compliant in virgin form
• Low smoke and flame characteristics

There are also some characteristics which can limit the use of PTFE, including relatively low mechanical properties and the inability of materials to adhere to the surface. Mechanical properties can be dramatically improved with the addition of fillers (glass, bronze, stainless steel, etc.) while etching the surface with sodium naphthalene or sodium ammonia will prepare the surface for adhesives. Note that the relatively low compressive strength of unfilled PTFE makes it an ex­cellent gasket material, while the filled and newer chemically modified resins greatly expand the usefulness in these applications.

PTFE molded and machined components. (Photo courtesy of Amicon Plastics.)

Processes and grades
PTFE is commonly processed by either ram extrusion (for rod and heavy walled tube) or by compression molding (sheet, heavy rod or tube and complex forms). Another common extrusion method, known as paste extrusion, is used to produce small diameter, thin walled tubing. With this method it is possible to produce tubing as fine as a human hair. Thin sheet is commonly skived, a process for cutting thin sheets (thicknesses typically below .010" up to .250") from a billet, which is a large cylindrical compression molded form.

Many people think of PTFE in two forms — mechanical and virgin. This is a slippery slope, since today what is typically sold as “mechanical” is probably lower grade virgin while some virgin resins may not be suitable for all applications, especially those of an electrical na­ture. It is far more prudent to classify PTFE according to ASTM D-3308-01, which contains four types and two grades, that can help to differentiate between end use suitabilities (e.g., semiconductor, elec­tri­cal, food use, gaskets, etc.). Your PTFE supplier can assist you or your customer in making the correct choice.

Physical, especially mechanical, properties can be greatly affected by fabrication techniques, including how the stock shape was processed as well as machining techniques employed. Poor techniques will result in finished parts with less than optimum properties.

It is especially important to choose correct tools and working speeds when machining PTFE components. Post an­nealing is sometimes required to ensure very close tolerances can be maintained.

Applications
As noted earlier, PTFE has found its way into all facets of our lives. Here are just a few examples:

• Seals for space shuttles
• Gaskets in chemical plants
• Expandable covering for “shunts” used to correct closed arteries
• Heat shrinkable tubing for aircraft wire insulation
• Coated glass tape for heat sealing
• Liners for chemical piping
• Valve and pump components

View the article online.

This article was written by Russ Consentino, CPMR, Plastic Solutions, Inc..

Diversity in PTFE

REPRINTED FROM THE IAPD MAGAZINE

Since its discovery in 1938, polytetrafluoroethylene (PTFE) has revolutionized the progress of modern man, as did the discovery of fossil fuels. PTFE has opened new doors for significant advancements in a variety of different technologies. From the aerospace industry to the food processing industry, PTFE has aided in many benefits to mankind. Most commonly known throughout the industry as Dupont’s brand name, Teflon®, PTFE has been commonly used in its virgin state. The material’s chemical inertness, low coefficient of friction and high temperature resistance are but a few advantages that it has to offer. As use of this material increased, so did the applications for it. Chemists soon began to add additional materials within PTFE to increase a variety of properties for their respective applications.

Virgin PTFE is normally white in color, but fillers alter the color as well as the properties.

Virgin grade
Processed in a variety of methods, normally through cold forming and a sintering process, this grade of PTFE resin of­fers several outstanding properties not found in other materials. The virgin resin has a large range of thermal applications since it can withstand temperatures from -450° to 500°F (-268° to +260°C), making it excellent for most cryogenic applications. The fluorine atom and the carbon atom are the building blocks of PTFE. When combined, they form the PTFE molecule and it develops a tendency to repel other molecules, thus giving the material the “non-stick” property it is well known for.

The virgin grade exhibits the lowest coefficient of friction of all solid materials making it extremely slippery. This tendency to repel also adds to the chemical inertness of the material. It has a chemical resistance to most chemicals except fluorochemicals and molten alkali metals. It has an extremely high electrical resistance and excellent dielectric strength. The virgin grade is also FDA approved for the food processing industry. Some virgin grades are also approved for insertion within the human body as possible prosthetics or even valves within an artificial heart.

Reprocessed grade
This grade is manufactured from pre-sintered PTFE shavings, scrap, etc. It exhibits most of the same properties that the virgin grade does but is subject to occasional contamination within the material. This is the grade of choice when cost is a ma­jor concern and cleanliness is not an issue.

Modified grade
The newest grade of fluoropolymers, modified PTFE offers the same thermal, chemical, low coefficient of friction and “non-stick” properties of a virgin PTFE but at the same time improves a variety of other properties. Modified PTFE offers a higher dielectric strength and improved performance in most electrical applications. Creep resistance and stiffness are improved making it an exceptional gasketing material.

The modified grade offers an excellent weldability characteristic that makes the fabrication of complex parts economically feasible. This improved characteristic also lowers the porosity and improves permeation resistance. Also, environmental safety concerns are improved because of lower fugitive emissions.

Glass fibers
Glass fibers are most commonly added to PTFE, normally in percentages that can range from 5 to 40 percent. The addition of the glass fibers improves the mechan­ical properties of PTFE as well as the compressive strength under load. The mater­ial’s wear resistance is also improved. The presence of glass fibers makes the material abrasive, so graphite is sometimes added to lower the coefficient of friction. The chemical and electrical properties virtually remain unchanged. The original chem­­ical inertness remains the same for the PTFE base, but the glass fibers can be attacked by alkali and hydrofluoric acid. The temperature range for the glass filled PTFE is -450° to +550°F (-268° to +287°C).

Carbon
Carbon can also be added to PTFE, normally within a percentage range of 10 to 35 percent. In most applications, graphite is also added to increase lubricity. The electrical properties are altered but the consistent chemical resistance of PTFE remains the same. Since the mechanical properties improve so does the wear re­sistance of the material. The improved wear resistance makes this a good choice for most dry and wet applications. The temperature range for the carbon filled PTFE is normally -320° to +500° F (-195° to +260°C).

Bronze powder
When bronze powder is blended with PTFE, it is normally in percentages ranging from 40 to 60 percent. Wear resistance is significantly improved along with a higher compressive strength and a lower creep value. The bronze within the PTFE is also a good thermal conductor, making it a good choice in high abrasive applications. The drawback to this material is the decrease in chemical resistance and the electrical properties. The bronze filler is subject to corrosive environments.

Graphite
Since graphite is a solid lubricant, it is sometimes added to PTFE, along with other fillers to lower the coefficient of friction. Frictional and wear properties are improved when the graphite is blended with other fillers within PTFE. When graphite is combined with another filler, it exhibits a good chemical resistance in most corrosive environments.

Molybdenum disulfide
Molybdenum disulfide, commonly known as “moly” when discussed as a filler, improves surface hardness of PTFE. It de­creases the coefficient of friction and is normally added to post-filled PTFE when the original filler makes the surface abrasive. It also has little to no effect on the chemical and electrical properties.

A variety of other fillers also exist, such as calcium fluoride, mica and stainless steel, just to name a few. The blending capabilities are endless as long as the re­quired application deems it necessary. The diversity found in PTFE, from the virgin grades through the filled grades, offer unique characteristics not found in other mate­rials, making it one of the most valued discoveries of our time.

View the complete article online.

This article was written by Carlos A. Baez, Plastomer Technologies: Texolon.

About PTFE

PTFE was the first fluorocarbon. It is the most chemically resistant plastic known. Only a few chemicals react with it. Its mechanical properties are low compared to other engineering plastics, but its properties remain at a useful level over a great temperature range — from -400 to 500°F (-240 to 260°C). Mechanical properties can be improved by the addition of fillers such as glass fiber, carbon, graphite, molybdenum disulfide and bronze. PTFE has excellent thermal and electrical insulation properties. And, it has a low coefficient of friction. It is difficult to make anything adhere to PTFE. A material may stick to it, but the material can be peeled off or rubbed off.

PTFE is not melt processible; instead it is processed by paste and ram extrusion or compression molding.

For more information on PTFE and other plastic materials, IAPD’s Introduction to Plastics is an invaluable training manual. Details about it and other IAPD educational resources are available online at www.iapd.org.

Test your knowledge

What do you know about PTFE? Answers are at www.iapd.org/popquiz.html.

1. What filler would you add to PTFE to best improve its load bearing properties?

1. Silicone
2. Glass
3. Moly
4. Carbon

2. What is the dynamic co-efficient of friction for PTFE?

1. 0.05
2. 0.15
3. 0.25
4. 0.35

Online plastic resources

Your IAPD Distributor is your choice in finding the right material for your application. Go to www.iapd.org to find a distributor in your area. You can search by company name, location or product category.

The IAPD Magazine web site at www.theiapdmagazine.com allows you to search by material, trade name and fabrication process. You can also search for fabrication capabilities.

Plastics Education in Philadelphia

Join IAPD in Philadelphia September 20 for a special one-day educational package specifically for engineers and specifiers to learn about innovations in both plastic materials and applications.

IAPD experts will first present the popular IAPD Plastics Applications Seminar, which offers a look at a wide variety of plastics applications. The seminar introduces and reintroduces materials in a variety of eye-opening applications. After the seminar, be sure to attend the IAPD Plastics Exhibition, a great opportunity to experience first-hand the best of the plastics stock shapes and pipe, valves and fittings distribution industry. Last year’s exhibition featured 70 exhibitors, and the show is the only one of its kind in North America.

Information is available online at www.iapd.org/new/pdf/events/ac/one_day.pdf.

© 2008

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Designing with Plastics is published by the International Association of Plastics Distribution. While every effort has been made to ensure accuracy, IAPD encourages you to verify information with a plastics distributor to ensure you select the correct plastic products to meet your needs.